Progress in the chemistry of higher fullerenes (>C70) suffers from the limited availability of these molecular allotropes of carbon, and even the amount of C60 (the first most abundant fullerene) and C70 (the second most abundant fullerene) produced by sooting flames is less than 9% of the soot mass (see A. Hirsch, M. Brettreich, Fullerenes, Chemistry and Reactions (Wiley-VCH, Weinheim, 2005).
Soxhlet-based solid-liquid extractions using toluene, evaporations and tedious chromatographic separations requiring large amounts of solvents are usually employed for the separation and purification of C60-C70 mixtures (see (a) D. H. Parker et al., Carbon 30, 1167 (1992); K. C. Khemani, M. Prato, F. Wudl, J. Org. Chem. 57, 3254 (1992); (b) W. A. Scrivens, P. V. Bedworth, J. M. Tour, J. Am. Chem. Soc. 114, 7917 (1992); (c) I. L. Isaacs, A. Wehrsig, F. Diederich, Helv. Chim. Acta 76, 1231 (1993); (d) N. Komatsu, T. Ohe, K. Matsushige, Carbon 42, 163 (2004)). For example, a complete protocol for fullerenes separation by column chromatography and HPLC has been reported by Diederich (C. Thilgen, F. Diederich, R. L. Whetten, Buckminsterfullerenes, 59 (1993)).
On the other hand, some separation methods based on selective complexation with Lewis acids have also been described in the background art (I. Bucsi, R. Aniszfeld, T. Shamma, G. K. S. Prakash, G. A. Olah, Proc. Natl. Acad. Sci. U.S.A., 91, 9019 (1994)) or host-guest chemistry, such as encapsulation into cyclodextrins (T. Anderson, K. Nilsson, M. Sundahl, G. Westman, O. Wennerström, J. Chem. Soc. Chem. Commun., 604 (1992)) or calix[8]arenes ((a) T. Suzuki, K. Nakashima, S. Shinkai, Chem. Lett., 699 (1994); (b) J. L. Atwood, G. A. Koutsantonis, C. IL. Raston, Nature 368, 229 (1994)). However, apart from their inherent elegance and esthetical appeal, these methods are unpractical because they are selective for the major component C60 but not for C70 or the higher fullerenes. Komatsu has reported a case of preferential precipitation of C70 over C60 with p-halohomooxacalix[3]arenes (N. Komatsu, Org. Biomol. Chem., 204 (2003)). Nevertheless, the release of the fullerene and simultaneous recovery of the valuable host from the complex proved difficult, due to its high stability.
Since the discovery of the high order fullerenes, ((a) Diederich, F.; Ettl, R.; Rubin, Y.; Wetthen, R. L.; Beck, R.; Álvarez, M.; Anz, S.; Sensharma, D.; Wuld, F.; Khemani, K. C.; Koch, A. Science, 1991, 252, 548-551.; (b) Ettl, E.; Diederich, F; Whetten, R. L Nature 1991, 353, 149-153.; (c) Diederich, F; Thilgen, C.; Whetten, R. L; Ettl, E.; Chao, I.; Alvarez, M. M Science 1991, 254, 1768-1770.; (d) Dennis, T. J. S.; Kai, T.; Tomiyama, T.; Shinohara, H. Chem. Commun 1998, 619-620), the isolation of said compounds is a challenging topic due to their low abundance, poor solubility and difficult separation. Until now, the most reliable method to purify high order fullerenes is HPLC. To reach sufficient purity, however, several cycles are required. The above mentioned drawbacks make the high order fullerenes very expensive. Therefore, high order fullerene chemistry has been poorly developed.
Other methods for the separation of high order fullerenes, based also on supramolecular interactions, have been recently described. For example, a new double calix[5]arene container successfully extracts higher fullerenes, especially C94 and C96, from fullerene mixtures. The syn-isomer of the double calix[5]arene selectively captures higher fullerenes from fullerene mixtures (Haino, T.; Fukunaga, C.; Fukazawa, Y. Org. Lett. 2006, 8, 3545-3548). By raising the temperature above 100° C., a conformational change to the anti isomer is promoted, thus releasing the captured high order fullerenes.
Other methods based on host-guest chemistry have been described, for example, Aida's cyclic dimers of zinc porphyrins (Shoji, Y.; Tashiro, K.; Aida, T. J. Am. Chem. Soc. 2004, 126, 6570-6571). These compounds are useful for the extraction of fullerenes≧C76 directly from fullerene mixtures, and upon several extractions, allow the enrichment of rare fullerenes C102-C110.
All the above described host-guest methods are beautifully designed but in all cases, chromatography is required in some step of the process.
The higher fullerene C84 is the third most abundant fullerene after C60 and C70 and has a total of 24 isomers (see a) Krätschmer, W.; Lamb, L. D.; Fostiropoulos, K.; Huffman, D. R. Nature 1990, 347, 354. b) Diederich, F.; Ettl, R.; Rubin, Y.; Whetten, R. L.; Beck, R.; Alvarez, M.; Anz, S.; Sensharma, D.; Wudl, F.; Khemani, K. C.; Koch, A. Science 1991, 252, 548). As a result of the very limited availability of pure C84 only very few reactions have been carried out on this fullerene and most of them have been performed on a very small scale with the goal to separate the different isomers or to test the reactivity of C84 (see a) Hawkins, J. M.; Nambu, M.; Meyer, A. J. Am. Chem. Soc. 1994, 116, 7642. b) Crassous, J.; Rivera, J.; Fender, N. S.; Shu, L. H.; Echegoyen, L.; Thilgen, C.; Herrmann, A.; Diederich, F. Angew. Chem. Int. Ed. 1999, 38, 1613. c) Wang, G. W.; Saunders, M.; Khong, A.; Cross, R J. J. Am. Chem. Soc. 2000, 122, 3216. d) Nuffer, R.; Bartl, A.; Dunsch, L.; Mathis, C. Synth. Met. 2001, 121, 1151. e) Wakahara, T.; Han, A. H.; Niino, Y.; Maeda, Y.; Akasaka, T.; Suzuki, T.; Yamamoto, K.; Kako, M.; Nakadaira, Y.; Kobayashi, K.; Nagase, S. J. Mater. Chem. 2002, 12, 2061. f) Darwish, A. D.; Martsinovich, N.; Taylor, R. Org. Biomol. Chem. 2004, 2, 1364). C84 has potential applications in the fields of nonlinear optics and superconductivity (see Shibata, K.; Kubozono, Y.; Kanbara, T.; Hosokawa, T.; Fujiwara, A.; Ito, Y.; Shinohara, H. Appl. Phys. Lett. 2004, 84, 2572) and can be used to develop organic solar cell devices ((a) Kooistra, F. B.; Mihailetchi, V. D.; Popescu, L. M.; Kronholm, D.; Blom, P. W. M.; Hummelen, J. C. Chem. Mater. 2006, 18, 3068. b) Anthopoulos, D.; Kooistra, F. B.; Wondergem, H. J.; Kronholm, D.; Hummelen, J. C.; de Leeuw, D. M. Adv. Mater. 2006, 18, 1679) and organic field effect transistors (see Shibata, K.; Kubozono, Y.; Kanbara, T.; Hosokawa, T.; Fujiwara, A.; Ito, Y.; Shinohara, H. Appl. Phys. Lett. 2004, 84, 2572).